Chemical-mechanical planarization ("CMP") processes remove material from the surface of a wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is exposed to an abrasive medium under controlled chemical, pressure, velocity and temperature conditions. One abrasive medium used in CMP processes is a slurry solution with small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. Another abrasive medium used in CMP processes is a generally planar planarizing substrate made from a relatively porous matrix material, such as blown polyurethane. To increase the abrasiveness of planarizing substrates, abrasive particles are embedded into the matrix material. Thus, when the wafer moves with respect to the abrasive medium, material is removed from the surface of the wafer mechanically by the abrasive particles in the substrate and/or the slurry, and chemically by the chemicals in the slurry.
In some new CMP processes, a planarizing liquid without abrasive particles is used with an abrasive substrate covered with fixed abrasive particles. The present invention is applicable to any CMP process that removes material from the surface of the wafer.
FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a wafer carrier 30, a planarizing substrate 40, and a planarizing solution 44 on the planarizing substrate 40. The planarizing substrate 40 may be a conventional polishing pad made from a continuous phase matrix material such as polyurethane, or it may be a substrate covered with fixed abrasive particles. The planarizing solution 44 may be a conventional CMP slurry with abrasive particles, or it may be a planarizing liquid without abrasive particles. An under-pad 25 is typically attached to an upper surface 22 of the platen 20, and the planarizing substrate 40 is positioned on the under-pad 25. In most conventional CMP machines, a drive assembly 26 rotates the platen 20 as indicated by arrow A. In another existing CMP machine, the drive assembly 26 reciprocates the platen back and forth as indicated by arrow B. The motion of the platen 20 is imparted to the substrate 40 through the under-pad 25 because the planarizing substrate 40 frictionally engages the under-pad 25.
The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows C and D, respectively.
In the operation of the CMP machine 10, the wafer 12 is positioned face-downward against the planarizing substrate 40 and at least one of the platen 20 or the wafer carrier 30 is moved relative to the other. As the face of the wafer 12 moves across the planarizing surface 42, the planarizing substrate 40 and the planarizing solution 44 remove material from the wafer 12.
One problem with CMP processing is that the throughput may drop, and the uniformity of the polished surface may be inadequate, because the condition of the polishing surface on the substrate deteriorates while polishing a wafer. The planarizing substrate surface deteriorates because waste particles from the wafer, substrate, and slurry accumulate on the planarizing substrate. The waste matter alters the condition of the polishing surface on the planarizing substrate causing the polishing rate to drift over time. The problem is particularly acute when planarizing doped silicon oxide layers because doping softens silicon oxide making it slightly viscous as it is planarized. As a result, accumulations of doped silicon oxide glaze the surface of the planarizing substrate with a glass-like material that substantially reduces the polishing rate over the glazed regions. Thus, it is often necessary to condition the substrate by removing the waste accumulations from its polishing surface.
Planarizing substrates are typically conditioned with an abrasive disk that moves across the planarizing substrate and abrades the waste accumulations from the surface of the substrate. One type of abrasive disk is a diamond-embedded plate mounted on a separate actuator that sweeps the plate across the substrate. Some substrate conditioners remove a thin layer of material from the deteriorated polishing surface in addition to the waste matter to form a new, clean polishing surface. Other substrate conditioners may use a liquid solution in addition to the abrasive disks to dissolve some of the waste matter as the abrasive disks abrade the planarizing substrate.
A more specific problem related to conditioning planarizing substrates is that conventional substrate conditioning devices and processes significantly reduce the throughput of CMP processing. During conventional conditioning processes with abrasive disks, large particles often detach from the abrasive disks and/or the substrate. The detached particles may scratch the wafer if the wafer is not removed from the substrate during conditioning, or if the substrate is not cleaned after conditioning. More specifically, therefore, conventional conditioning processes with abrasive disks reduce the throughput of CMP processing because removing the wafer from the substrate and cleaning the substrate after conditioning requires down-time during which a wafer cannot be planarized.
In light of the problems associated with conventional abrasive conditioning processes, it is desirable to chemically condition the substrate by dissolving an adequate amount of waste matter from the substrate without mechanically abrading the waste matter or the planarizing substrate. Chemical conditioning of planarizing substrates is a new and promising method to increase the throughput of the finished wafers. Yet, as explained below, it is difficult to chemically condition a planarizing substrate in situ and in real-time while a wafer is being planarized in some circumstances.
One problem of chemically conditioning a planarizing substrate in situ and in real-time is that the conditioning solution mixes with the planarizing solution. The conditioning solution accordingly dilutes the concentration of abrasive particles in a conventional slurry, and it may react adversely with the chemicals in the planarizing solution. Another problem with chemically conditioning the planarizing substrate is that the conditioning solution may not be chemically compatible with the wafer. Therefore, it would be desirable to develop an apparatus and a method for chemically conditioning a planarizing substrate while a wafer is being planarized.